All units used in the book are included in this glossary, and are tinted bold the first time they appear within the text of the book. Tinted bold text in this glossary refers to terms elsewhere in this glossary or to terms in the glossary of terms.
K Kelvin, a unit of measurement for temperature. One degree kelvin = one degree Celsius, but zero degrees kelvin (0K) is absolute zero. 0°C is approx. 273K.
Km/W Kelvin metres per watt. The unit of thermal resistivity.
kWh Kilowatt hour, a unit of energy. For example, a one-kilowatt electric fire left on for one hour would use 1kWh (1,000W) of energy. 1kWh = 3.6MJ (megajoules).
kWh/m².a Kilowatt hours per square metre per annum. Measures energy used annually per square metre of usable or treated floor area (TFA). This is one of the key units of measure in Passivhaus. By defining energy use in terms of each square metre of floor area, it allows us to make a meaningful comparison of the energy use of buildings of different sizes. See annual [specific] space heat demand.
kWp Kilowatt peak. A measure of the maximum power output of photovoltaic (solar) panels.
m³/hr Cubic metres [of air] per hour. The unit of measure used to describe ventilation rates. Passivhaus ventilation calculations are based on providing 30m³ per person per hour. This rate is necessary to keep carbon dioxide (CO2) levels well below 1000 parts per million (ppm).
m³/hr/m² Cubic metres [of air] per hour per square metre [of thermal envelope area]. The unit of measure of air permeability.
MJ/kg Megajoules per kilogram. Unit of measure of embodied energy.
MJ/m² Megajoules per square metre. A megajoule (MJ) is a unit of energy, equivalent to 1,000,000J (joules). 3.6MJ = 1kWh.
MNs/g Meganewton seconds per gram. The unit of measure of vapour resistance (G-value). See vapour permeability.
MNs/gm Meganewton seconds per gram metre. The unit of measure of vapour resistivity (r-value). See vapour permeability.
Pa Pascal, the SI (International System of Units) measurement of force per unit area. 1Pa is 1 newton per square metre.
W Watt, a unit of power. The rate at which energy transfers (is ‘consumed’ or ‘generated’). For example, a typical low-energy lightbulb uses 15W; an electric kettle 3kW (kilowatts, i.e. 1,000W). 1W = 1 joule per second.
W/K Watts per degree kelvin [temperature difference between inside and outside the thermal envelope]. Used to quantify chi-value (c) in point thermal bridges.
Wh/K per m² (as cited in the PHPP) Watt hours per kelvin per square metre [of treated floor area (TFA)]. 1 watt hour (Wh) is one-thousandth of a kWh (kilowatt hour): for example, a 1W LED light bulb left on for 1 hour would use 1Wh of electricity.
Wh/m³ Watt hours per cubic metre [of air moved]. Used by the Passivhaus Institut (PHI) to measure the electrical efficiency of MVHR units. 1Wh (watt hour) is one-thousandth of a kWh.
W/m² Watts per square metre [of treated floor area (TFA)]. Used to measure heat load.
W/m²K Watts per square metre [of the material/ assembly in question] per degree kelvin [temperature difference between inside and outside the thermal envelope]. Used to quantify U-value.
W/mK Watts per metre per degree kelvin. For linear thermal bridges, this is watts per metre length of the thermal bridge per degree kelvin temperature difference. (Used to measure psi-value [y].) For thermal conductivity this is watts per metre thickness/depth of material per degree kelvin temperature difference. (Used to measure lambda value [λ].) In both cases the temperature difference measured is that between inside and outside the thermal envelope.